The present invention relates to a separator for a battery. More particularly, the present invention relates to a separator for a battery having high safety, a battery using the same and to a process for preparing the separator.
There is a growing demand for downsizing and lightening of portable electric appliances and the achievement greatly depends upon improvement of battery performance. There have been various development and improvement of batteries in order to meet the demand. Performance required to a battery includes high voltage, high energy density, safety, variety of shape and the like. A lithium ion battery is a non-aqueous electrolytic solution battery, which is expected to achieve high voltage and high energy density, and active improvement is going on even at present. Also, there has been carried out research for lithium metal batteries which is expected to have further higher energy density.
These non-aqueous batteries comprise a positive electrode, a negative electrode and an ion conductive layer sandwiched between the both electrodes as major components. In lithium ion batteries practically used these days, material in a shape of a plate obtained by applying powder such as lithium cobalt oxide to a current collector is used as an active material for the positive electrode. In the same manner, material in a shape of a plate obtained by applying powder such as carbon material to a current collector is used as an active material for the negative electrode. In order for these electrodes to function as a battery, it is necessary to have a layer between the both electrodes through which lithium ions can move and which has no electric conductivity. Generally, a porous film separator such as polyethylene is used as the ion conductive layer. It is interposed between the both electrodes and non-aqueous solution is filled to constitute the ion conductive layer.
As well as a function to electrically insulate the both electrodes, the separator also has safety improvement ability that it melts and makes minute holes inside the separator smaller, leading to cutting off ion conductivity in temperature increase due to unusual conditions such as short circuit. However, the above separator had a problem that not only the minute holes were closed but also at a temperature higher than a certain degree, the separator itself melted to cause deformation of the separator such as shrink and generation of holes due to melting and insulation was broken. At this time, since large short-circuit current is generated between the positive electrode and the negative electrode, there has been a problem that the temperature of the battery further increases due to exothermic reaction, leading to further increase of short-circuit current.
In Japanese Unexamined Patent Publication No. 241655/1998, there is disclosed a separator obtained by solidifying insulating inorganic particles with a binder. However, such a separator had problems that it could not control ion conductivity at a high temperature and that preparation of the battery became complicated.
The present invention has been carried out in order to solve the above problems. The object of the present invention is to provide a highly safe separator capable of efficiently cutting off ion conductivity at a high temperature without the risk that the separator melts to cause break of insulation, a battery using the same and a process for preparing the above separator.
The first separator for batteries of the present invention comprises a first porous layer containing a thermoplastic resin as a main component and a second porous layer placed on the first porous layer which has higher heat resistance than that of the first porous layer.
According to this, there is an effect that highly safe separator capable of efficiently cutting off ion conductivity at a high temperature can be obtained without the risk of its melting which causes break of insulation.
The second separator for batteries of the present invention has structure that the second porous layer having higher heat resistance than that of the first porous layer is interposed by the first porous layers which contains a thermoplastic resin as a main component.
According to this, there is an effect that highly safe separator capable of efficiently cutting off ion conductivity at a high temperature can be obtained without the risk of its melting which causes break of insulation. Also, there is an effect that a separator which can easily handle is prepared.
The third separator for batteries of the present invention is that in the first separator, the second porous layer comprises fine particles having higher heat resistance than that of the first porous layer.
According to this, there is an effect that thin and fine porous layer can be easily formed and that it gives flexibility to the second porous layer.
The first battery of the present invention comprises a separator interposed between a positive electrode and a negative electrode, wherein the separator comprises a first porous layer containing a thermoplastic resin as a main component and a second porous layer having higher heat resistance than that of the first porous layer laminated on the first porous layer.
According to this, there is an effect that a highly safe battery can be obtained since current increase between the electrodes can be controlled when a temperature is increased by exothermic reaction due to short-circuit.
The second battery of the present invention comprises a separator interposed between a positive electrode and a negative electrode, wherein the separator has structure that the second porous layer having higher heat resistance than that of the first porous layer is interposed by the first porous layers which have a thermoplastic resin as a major component.
According to this, there is an effect that a highly safe battery can be easily obtained even when a temperature is increased.
The first process for preparing the separator of the present invention comprises forming a second porous layer by applying fine particles whose heat resistance is higher than that of the first porous layer onto the first porous layer containing a thermoplastic resin as a main component.
According to this, there is an effect that highly safe separator capable of efficiently cutting off ion conductivity at a high temperature can be easily prepared without the risk that the separator melts to break insulation.